Driver circuit for organic electroluminescent display

ABSTRACT

A simple driver circuit used for an organic electroluminescent (EL) device comprises a transparent substrate on which an anode, at least one organic layer, and a cathode are formed. The anode is made of a transparent conductive material. The cathode is made of a conductive metal material. The driver circuit has a resistor electrically connected with the EL device to form a series circuit. A DC power supply is connected with this series circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit for driving an organicelectroluminescent (EL) circuit.

2. Description of the Related Art

An organic electroluminescent (EL) device comprising a transparentsubstrate on which an anode, at least organic layer, and a cathode areformed is disclosed, for example, in Japanese Patent Publication No.32307/1994. The anode is made of a transparent conductive material. Theorganic layer consists either of an organic emissive layer or of amultilayer structure containing organic layers such as a hole injectionlayer. If necessary, a hole transport layer is added to the multilayerstructure. The cathode is made of a conductive metal material.

In this organic EL device, a DC voltage of several volts to tens ofvolts is applied between the anode and the cathode to cause the organiclayer to emit light. The organic EL device has the advantage that it canbe driven at a lower driving voltage than other kinds of EL devices.

Generally, a driver circuit for this kind of organic EL device iscomposed of a constant-current circuit. A known circuit is shown in FIG.7, where the organic EL device is indicated by 01 and has an anode 02and a cathode 03. A DC power supply supply 06 such as a battery isconnected with the EL device 01 via a series combination of a transistor04 and a resistor 05. A series combination of a zener diode 07 and aresistor 08 is connected in parallel with the DC power supply 06. Theneutral point of this series circuit is connected with the positiveterminal of an operational amplifier 09. The negative terminal of theamplifier 09 is connected with the junction of the amplifier 09 and thetransistor 04. This driver circuit limits the current supplied to theorganic EL device 01 to a certain value at all times.

This driver circuit for an organic EL device needs numerous electroniccomponents including transistor 04 and operational amplifier 09. Wherenumerous display segments are formed on the anode 02, or where pluralanodes 02 and plural cathodes 03 intersect to form a matrix structure,it is necessary to prepare the aforementioned electronic components foreach of the electrodes. This increases the number of components, leadingto an increase in the cost.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an organic. ELdevice driver circuit free of the foregoing problems.

A DC power supply (4) is connected with an organic electroluminescent(EL) device (1) having an anode (2) and a cathode (3). Thus, a drivercircuit that is simple and made up of an unincreased number ofcomponents is obtained.

The organic EL device (1) and a resistor (5) are electrically connectedto form a series circuit. The DC power supply (4) is connected with thisseries circuit. In particular, the resistor (5) is connected with theanode (2) of the organic EL device (1) to form the series circuit withwhich the DC power supply (4) is connected.

In this way, the driver circuit for the organic EL device is simple instructure and made up of an unincreased number of components.Furthermore, the driver circuit is insusceptible to ambient temperaturevariations.

Other objects and features of the invention will appear in the course ofthe description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a driver circuit in accordance with a firstembodiment of the present invention;

FIG. 2 is a graph showing the voltage-current characteristics of thedriver circuit shown in FIG. 1;

FIG. 3 is a graph showing the temperature-brightness change percentcharacteristics of the driver circuit shown in FIG. 1;

FIG. 4 is a diagram of a driver circuit in accordance with a secondembodiment of the present invention;

FIG. 5 is a graph showing the voltage-current characteristics of thedriver circuit shown in FIG. 4;

FIG. 6 is a graph showing the temperature-brightness change percentcharacteristics of the driver circuit shown in FIG. 4; and

FIG. 7 is a diagram of a conventional driver circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention are hereinafterdescribed with reference to the accompanying drawings. FIGS. 1-3 pertainto a driver circuit in accordance with a first embodiment of theinvention. In FIG. 1, an organic electroluminescent (EL) device isgenerally indicated by numeral 1 and has an anode 2 and a cathode 3. ADC power supply 4 is connected in series with the organic EL device 1.

Specifically, the organic EL device 1 is driven by a constant voltagefrom the DC power supply 4 to emit light. The number of components isreduced compared with the conventional driver circuit (FIG. 7) driven bya constant current. The novel driver circuit for the organic EL deviceis simple in structure and made up of an unincreased number ofcomponents.

It is known that the current flowing through the organic EL device 1driven by the driver circuit described above varies with the voltage ofthe DC power supply 4, as shown in FIG. 2, and that the current varieswith the ambient temperature even if the voltage is kept constant. It isalso known that the brightness of emission from the organic EL device 1is roughly proportional to the current flowing through the EL device 1.Therefore, in this configuration, if the same voltage is applied, thebrightness varies greatly in response to ambient temperature variationsas shown in FIG. 3.

It is now assumed that the ambient temperature satisfies the relationT0<T1 (room temperature)<T2. Let V be the driving voltage from the DCpower supply 4. The operating point of the organic EL device is given bythe intersection of the voltage-current characteristic curve (A, B, or Cin FIG. 2) of the EL device 1 with the driving voltage V at temperatureT0, T1, or T2. If the voltage V is kept constant, the current variesfrom I₀ to I₂ (within the range X in FIG. 2). Consequently, thebrightness is varied greatly (see FIG. 3).

A driver circuit in accordance with a second embodiment of the inventionis shown in FIGS. 4-6. The second embodiment is an improvement on thefirst embodiment. Note that like components are indicated by likereference numerals in various figures. In FIG. 4, an organicelectroluminescent (EL) device 1 and a resistor 5 are electricallyconnected to form a series circuit. A DC power supply 4 is connectedwith this series circuit. In particular, the resistor 5 is connectedwith the anode 2 of the EL device 1, thus forming the series circuit.The DC power supply 4 is connected with this series circuit.

The driver circuit of this construction shows voltage-currentcharacteristics shown in FIG. 5. The operating point of the EL device 1is given by the intersection of the voltage-current characteristic curveA, B, or C of FIG. 5 at different ambient temperature T0, T1, or T2 withthe voltage-current characteristic line D of the resistor 5 in FIG. 5.It follows that the current varies from I₀′ to I₂′ (within range Y) at agiven voltage V. At ambient temperature T1, voltage Ve is applied to theorganic EL device 1, and voltage Vr is applied to the resistor 5. Thegradient of the voltage-current characteristic line of the resistor 5 isdetermined by 1/R, where R is the resistance of the resistor 5.

In this embodiment, the brightness of the organic EL device 1 varies toa lesser extent than in the previous embodiment because of the relationX>Y (see FIG. 6). In the second embodiment, the resistor 5 is requiredto be connected with only one of the anode 2 and the cathode 3 of theorganic EL device.

The present invention can provide a driver circuit that drives anorganic EL device, is simple in structure, and is made up of anunincreased number of components.

What is claimed is:
 1. A driver circuit for use with an organicelectroluminescent (EL) device having a transparent substrate, an anodemade of a transparent conductive material and formed on said substrate,at least one organic layer formed on said anode, and a cathode made of aconductive metal material and formed on said organic layer, said drivercircuit comprising: a DC power supply connected in series with saidorganic EL device to drive it with a constant voltage.
 2. A drivercircuit for use with an organic electroluminescent (EL) device having atransparent substrate, an anode made of a transparent conductivematerial and formed on said substrate, at least one organic layer formedon said anode, and a cathode made of a conductive metal material andformed on said organic layer, said driver circuit comprising: a resistorelectrically connected with said organic EL device to form a seriescircuit; and a DC power supply connected with said series circuit.
 3. Adriver circuit for use with an organic electroluminescent (EL) devicehaving a transparent substrate, an anode made of a transparentconductive material and formed on said substrate, at least one organiclayer formed on said anode, and a cathode made of a conductive metalmaterial and formed on said organic layer, said driver circuitcomprising: a resistor connected with at least one of said anode andsaid cathode of said organic EL device to form a series circuit; and aDC power supply connected with said series circuit.